The rapidly fading optical afterglow of GRB 980519

GRB 980519 had the most rapidly fading of the well-documented gamma-ray bur st (GRB) afterglows, consistent with t(-2.05+/-0.04) in BVRI as well as in X-rays during the 2 days in which observations were made. We report VRI obs ervations from the MDM 1.3 m and WIYN 3.5 m telescopes, and we synthesize a n optical spectrum from all of the available photometry. The optical spectr um alone is well fitted by a power law of the form nu(-1.20+/-0.25), with s ome of the uncertainty due to the significant Galactic reddening in this di rection. The optical and X-ray spectra together are adequately fitted by a single power law, nu(-1.05+/-0.10). This combination of steep temporal deca y and flat broadband spectrum places a severe strain on the simplest afterg low models involving spherical blast waves in a homogeneous medium. Instead , the rapid observed temporal decay is more consistent with models of expan sion into a medium of density n(r) proportional to r(-2), or with predictio ns of the evolution of a jet after it slows down and spreads laterally. The jet model would relax the energy requirements on some of the more extreme GRBs, of which GRB 980519 is likely to be an example because of its large g amma-ray fluence and faint host galaxy.